450 CEUSTACEA. 



even be reduced to an unsegmented stump-shaped appendage 

 ( Lcemodipoda) . 



The nervous system consists of a cerebral ganglion and a ventral gan- 

 glionic chain, which is most distinctly composed of two lateral halves. 

 In the Isopoda there is also an unpaired visceral nerve. The two eyes 

 are always sessile, compound eyes, with smooth or facetted cornea; 

 they are never stalked. Delicate olfactory fibres are often present 

 on the anterior antennae, and are especially numerous in the male 

 sex. 



The alimentary canal begins with a short O3sophagus, which passes 

 upwards to open into a wide crop, supported by firm horny bands 

 and often armed with strong chitinous plates. The crop leads into a 

 long intestine provided with two or three pairs of tubular hepatic 

 glands. The rectum, which may possess one or two tubular appen- 

 dages (probably urinary), opens at the posterior end of the body. 



The antennal gland opens on the basal segment of the posterior 

 antenna, often upon a conical protuberance. 



Vascular system. A heart is always present as the central organ 

 of the circulation. It may either have the form of a tube extending 

 along the whole length of the thorax (Amphipoda) ; or it may be 

 saccular and placed in the abdomen (Isopoda). In the first case the 

 gills are placed on the thoracic feet as tubular appendages ; in the 

 latter, on the other hand, they are placed on the abdomen. From the 

 heart the blood passes through an anterior and posterior aorta, and 

 usually through lateral arteries. The vessels conduct the blood into 

 the body cavity, whence it returns in regular streams to the lateral 

 paired slits of the heart. 



Generative organs. The Arthrostraca, are of separate sexes. The 

 males are frequently distinguished from the females by the modifica- 

 tion of certain parts of the appendages to form prehensile organs, by 

 a greater development of olfactory hairs on the anterior antennae, and 

 by the position of the sexual and copulatory organs. It is rare to 

 find a strongly marked dimorphism of the sexes (Bopyrus, Praniza). 

 The generative organs open either at the posterior part of the thorax 

 or at the base of, the abdomen ; the female always on the ante- 

 penultimate pair, the male on the last pair of the thoracic appen- 

 dages or between the first of the abdomen (Iso%)oda). The ovaries 

 are two simple or branched tubes with the same number of oviducts. 

 The testes similarly seem to be composed of one (Amphi%)oda) or 

 more (3) pairs of tubes (Isopoda), the efferent ducts of which (vasa 

 deferentia) either remain separate or unite to form a copulatory 



